In recent years an increasing appreciation has developed for the ability to alter a wide variety of material properties simply by reducing the material domain size to such an extent that quantum (i.e. molecular or atomistic) effects are non-negligible. Semiconductor materials have generally been the subject of much of this investigation, particular attention has been paid to silicon. Silicon quantum size effects have been exploited to alter the bulk properties of: photoluminescence (H. Takagi, H. Ozawa, Y. Yamozaki, A. Ishizaki and T. Nagakiri, Appl. Phys. Lett. 56, 2379 (1991)); melting and sintering (A. N. Goldstein, Appl. Phys. A 62, 33 (1996); U.S. Pat. No. 5,576,248); band gap energy (S. Furakawa and T. Miyasato, Superlattices and Microstructures 5, 317(1989)); physical strength of derivative ceramics (D. T. Castro and J. H. Ying, Matls. Sci. and Eng. A204, 1995); and phosphorescence (U.S. Pat. No. 5,446,286 and U.S. Pat. No. 5,433,489). Owing to the broad range of uses of silicon in modem technology, modifications of its properties has far reaching consequences in industrial sectors including: electronics, aerospace, computers, energy and sensors. Efforts to extend the range of properties of silicon have lead by extension to the other Group IV materials germanium, and to a lesser extent tin. Germanium and tin are advantageous over comparable alternative materials because the well established processing techniques associated with silicon are amenable to use with these other Group IV elements.
Investigation of the quantum size effect in the Group IV elements silicon, germanium and tin have been hampered by the inability to produce macroscopic quantities of such particles. The desired samples consist of particles that are: monodisperse; of tunable domain size on the dimensional scale wherein quantum size effects are observed, typically from 1 to 20 nanometers; amenable to variations in the surface passivating functionalities, which serve to prevent agglomeration and formation of bulk domains; produced in macroscopic quantities; dispersible in a gaseous or liquid carrier, thereby facilitating isolated particle behavior; and amenable to doping with various ions and molecular dyes common to the art. In part due to the covalent, nonpolar nature of bonding between like atoms of Group IV elements, especially silicon, the metathesis reactions used to produce nanocrystals of Group II-VI semiconductors are inoperative. A nanocrystal is defined as a crystalline particle having cross sectional dimensions ranging from about 1 to 100 nanometers (hereafter nanometers are designated as "nm"). Metathesis reactions become increasingly difficult as the polarity of the resulting bond decreases. While Group II-VI nanocrystal materials such as CdS are readily produced from molecular precursors in an aqueous liquid phase solution (M. L. Steigerwald et al., J Am. Chem. Soc. 110, 3046 (1988); D. J. Norris, A. Scara, C. B. Murray, and M. G. Bawendi, Phys. Rev. Let. 72, 2612 (1994; H. J. Watzke and J. H. Fendler, J Phys. Chem. 91, 6320 (1988)), the isoelectronic Group III-V nanocrystal, GaAs is produced only under air and water sensitive reaction conditions (M. A. Olshavsky, A. N. Goldstein and A. P. Alivisatos, J Am. Chem. Soc. 112, 9438 (1990)). Since there is no analogous metathesis reaction for the Group IV elements silicon, germanium and tin, alternative particle synthesis techniques have been devised.
Polycrystalline silicon and nanocrystalline silicon have been produced by a variety of well known techniques such as evaporation (S. Ijima, Jap. J Appl. Phys. 26, 357 (1987)), gas phase pyrolysis (K. A Littau, P. J. Szajowski, A. J. Muller, A. R. Kortan, L. E. Brus, J Phys. Chem. 97, 1224 (1993)), gas phase photolysis (J. M. Jasinski and F. K. LeGoues, Chem. Mater. 3, 989 (1991);), electrochemical etching (V. Petrova-Koch et al., Appl. Phys. Lett. 61, 943 (1992)), plasma decomposition of silanes and polysilanes (H. Takagi et al, Appl. Phys. Lett. 56, 2379 (1990)), high pressure liquid phase reduction-oxidation reaction (J. R. Heath, Science 258, 1131 (1992)) and most recently by refluxing the zintyl salt, KSi with excess silicon tetrachloride in a solvent of glyme, diglyme, or THF under nitrogen (R. A. Bley and S. M. Kauzlarich, J Am. Chem. Soc., 118, 12461(1996)). While each of the above mentioned techniques satisfies some of the criteria desired for particle samples, none of these techniques is inclusive of all the desired properties.
Techniques for producing particles that are inclusive of all the desired properties are known to the art of producing metal colloids. Classical colloid chemistry teaches the that the addition of dispersants, passivating agents, and peptizing agents serve to stabilize the thermodynamically metastable colloidal domains. By way of example, such stabilizing agents have illustratively included soaps and detergents of fatty acids, resins, polyphosphates, organic polymers, chemically bonded small organic molecules of molecular weight less than about 500 and containing a nonfacile heteroatom such as O, N, S, P and the like, which serves to adsorb the molecule to the particle; clays and biopolymers such as albumin. Since metathesis reactions are disfavored for producing metal colloids for reasons similar to those for silicon, reduction-oxidation reactions are typically utilized to produce metal sols.
J. R. Thomas, U. S. Pat. No. 3,167,525 which is incorporated herein by reference, teaches electropositive metal dispersions stabilized by polymers are produced in liquid solution over a range of sizes and concentrations by decomposition of an organometallic precursor in which all bonds of the metal are to carbon. This method generally combines an alkylated or carbonylated metal with a class of stabilizing polymer in a hydrocarbon or ether solvent. The solution reaction is then initiated by exposure to heating, actinic or ultraviolet light.
Organosilanes have a well established chemistry as decomposition precursors to silicon. The gas-phase, ultraviolet (UV) photolysis the of organosilanes, tetraethyl- and tetravinyl-silane shows a stepwise elimination of 2-carbon aliphatics, resulting in the production of SiH.sub.4, using the ArF laser line at 193 nm. Due to the low UV absorption cross section at 193 nm, silane is slow to decompose to silicon. J. Pola, J. P. Parsons and R. Taylor, J Orgmet. Chem. 489,C9-C11 (1995), which is incorporated herein by reference. Phenylsilanes are known to decompose under similar conditions by a two-channel process, yielding PhSiH+H and SiH2+PhH. J. E. Baggott, H. M. Frey, P. D. Lightfoot and R. Walsh, Chem. Phys. Lett. 125, 22 (1986); which is incorporated herein by reference.
The UV absorption cross section of silanes and disilanes of the form R.sub.4 Si and R.sub.6 Si.sub.2, respectively, generally increases at longer wavelengths with increased molecular weight and increased bond delocalization. Silanes also show a shift to longer wavelengths in the absorption cross section upon dissolution in a solvent. The shift to longer wavelengths is more pronounced in a polar solvent, such as ethanol as compared to nonpolar solvent such as isooctane. The "red" shift in absorption is associated with an increased stability. G. B. Butler and B. Iachia, J Macromol. Sci. -Chem., A3(5) (1969) 803; which is incorporated herein by reference.
Organosilanes also form polysilanes under certain reaction conditions. Common reactions leading to polysilanes include condensation with alkali metals, dehydrogenative coupling in the presence of a suitable catalyst, and strained cyclosilane ring opening polymerization. A general discussion of polysilane chemistry is included in Inorganic Polymers by J. E Mark, H. R. Allcock and R. West, Prentice Hall, Englewood Cliffs, N.J., 1992, Chapter 5; which is incorporated herein by reference. Polysilanes generally undergo chain scission upon exposure to ultraviolet light. Under exhaustive UV exposure disilane is a major scission product of polysilane reaction. However, aryl, and carbon-carbon double bond containing groups bound to the silicon atoms of a polysilane are known to undergo free radical cross-linking, as well as chain scission under UV exposure, thereby creating silicon atoms which are indirectly linked by means of the aliphatic units.
While the preceding discussion details the chemistry of organosilanes, analogous chemistries exist for organogermanium and organotin compounds. The photochemical decomposition of such organometallics are shown in the instant invention to be amenable to the production of particles of silicon, germanium and tin with properties heretofore unattainable. While the photolysis of the organometallics has often been carried out with UV photon sources, infrared multiphoton decomposition of tetra-ethylsilane, -germanium and -tin has been reported to initiate with the cleavage of the (Group IV element)- carbon bond, as reported by T. Majima, K. Nagahama, T. Ishii, RezaKagaku Kenkyu, 9, 17 (1987).
Thus, it is an object of the present invention to produce novel forms of the individual Group IV elements silicon, germanium, and tin heretofore unattainable; wherein the domain size of the element is sufficiently small so as to show quantum size effects as evidenced by atomistic, non-bulk electronic or physical properties being evident in the particles as non-bulk: energy band gap, photoluminescence, or melting properties; the particle surface is chemically adjustable and both the average domain size and the size distribution are adjustable. It is a further goal of the invention to provide a generic synthesis process by which colloidal and or polyatomic forms of silicon, germanium and tin are produced in a liquid phase solvent under conditions of about Standard Pressure of one atmosphere and below.
SUMMARY OF THE INVENTION
According to the present invention the Group IV elements silicon, germanium and tin form nanocrystalline particles of the separate elements, by the photolysis of organometallic precursors of the aforementioned elements in a solvent that further contains a passivating agent to stabilize the particles upon formation. Ultraviolet light is the preferred photolysis light source. The factors involved in selecting an organometallic precursor which is operative in the instant invention include: absorption cross section at the incident light source wavelength(s), solubility in the solvent, and chemical driving force for the removal of precursor organic fragments. Photolysis of the Group IV precursor of the instant invention in a suitable reaction vessel creates nanocrystalline domains of the Group IV element. An organometallic precursor of second element, which is not restricted to Group IV is optionally added to the reaction to create doped nanocrystals. These particles are also amenable to incorporation of dopant ions known to the art of bulk Group IV technology, in order to modify illustratively the electronic, luminescent and physical properties of the nanocrystals. A passivating agent serves to keep the nanocrystal particles from aggregating and additionally to exert control over particle solubility and surface chemistry. The photolysis reactions described herein are generally performed under an inert atmosphere of about one atmosphere pressure or below.
The instant invention creates particles of silicon and germanium that are themselves novel. Particles are formed which have a size distribution of less than twenty percent of the particle diameter in the size regime wherein quantum size effects are observed. Quantum size effects are defined to include non-bulk: energy band gap, or photoluminescence or melting behavior observed in particles of a given size. The particles are optionally stable in solution phase and have a selectively controllable surface chemistry via the surface passivating agents employed. Furthermore, the resulting particles are generally free from reaction by-products.